Clarify methods used to create spline of light environment and calculate assimilation

[dfalster]

This PR attempts to simplify the language and methods used to both

• calculate the splines of light availability within the canopy
• calculate a plants assimilation, integrating over these splines.

Each of the above uses some numerical methods, and the language used for these was not very clear. There were also some options that were not really needed.

Changes include

• renamed things to do with canopy -> light_availability (canopy was too vague)
  • [x] canopy.h -> resource_spline.h (generalised) -[x] also pass in full function to integrator, so can simplify and generalise Resource_spline (though currently limited on scale 0-1, see #385 )
  • [x] canopy.compute_canopy -> light_availability.compute_environment (consistent with names elsewhere)
  • [x] control variables canopy_light_* -> light_availability_spline_*
  • [x] canopy::set_fixed_canopy -> light_availability::set_fixed_value
  • [x] get_canopy_at_height & canopy_openness -> get_value_at_height
• removed
  • [x] options for adaptive integration of assimilation, reusing intervals
  • [x] assimiolation class, preferable to call integrator straight from within strategy

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[dfalster]

Thanks Andrew, Isaac: Good feedback. I addressed those concerns in commit above. Once it passes I'll merge.